Heat treatment of magnetic materials



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UNITED `s'mrlas "PATENT OFFICE LOUIS W. MGKEEHAN, F MAPLEWOOD, NEWJERSEY, ASSIGNOR TO BELL TELEPHONE LABORATORIES, INCORPORATED, OF NEWYORK, N. YI, A CORPORATION OF NEW YORK HEAT TREATMENT lOIE MAGNETICMATERIALS Application led October 30, 1926. Serial No. 145,124.

This invention'relates to magnetic materials, and particularly toproducing desirable magnetic characteristics in nickel-iron alloys.

A feature of the invention resides in treating certain magneticcompositions, particularly nickel-iron alloys containing fram 60% to 80%of nickel, to give them substantially constant permeability withaccompanying lovsT hysteresis loss at low magnetizing forces of theorder employed in signaling.

Recent discoveries have taught that magneti materials containing nickeland iron as the principal ingredients can be given very highpermeability at low magnetizing forces and a low hysteresis loss ascompared with iron. Examples `of such materials and methods ofheat-treatingthem' to obtain the desired properties are disclosed in U.S.-

Patent No. 1,586,884 to G. .W. Elmen, issued June 1, 1926.

The usual method of preparing nickel-iron compositions comprises castingthe elements into an alloy, alternately 4working and annealing the alloyuntil the desired shape or form is obtained, and then heat-treating itin its finalform. Many of the alloys con-- taining nickel and iron aresensitive to strain, the permeability changing markedly 3o whenstress isapplied. One reason for heattreating the materials after they are intheir final form is to .remove unfavorable residual strains.

' The heat treatment to which these materials are subjected dependslargely upon `the characteristics whiclrare desired in the linishedproduct. For example,'one treatment which may -be employed eectively todevelop high permeability consists in heating the material to atemperature of about 1200 C. for about twenty hours in a vacuum furnace,and then allowing the material to cool downto room temperature with thefurnace. Another treatmentl resulting in high permeability consists in,heating the material to about 875 C., by passing an electric currentthrough it for several minutes, then cutting off the current andallowing it to cool. The rate of cooling in 'the second case, of course,is much more rapid than in the first. Other methods of heattreating thematerials are also employed.

The present invention is concerned particularly with nickel-iron alloyscontaining from about to 80% nickel. When alloys within this range aregiven the heat treatment hereinafter described, the permeability,becomes substantially constant for ma nevtizing forces up to at least.1 gauss. T ese alloys, when treated according to the invention, exhibita much higher initial permeability than iron as well as substantiallyconstant permeability through the range of eld strengths ordinarilyemployed in signaling circuits.

The characteristics of substantially constant permeability andaccompanying low hysteresis loss which are exhibited by these alloyswhen subjected to different heat treati ments, showing the greaterconstancy of permeability obtainable by means of this invention. In eachcase the magnetizing forces are plotted in c.` g. s. units. The magneticalloys hereinafter considered are prepared by meltingthe constituentstogether in the desired proportions in an induction furnace. Goodcommercial gradesof these materials are suitable. The molten metal isthen cast into rods or bars which are subsequently rolled or drawn intoany desired shape. For convenience in treating and testing samples ofthese alloys they are sometimes worked into 40 mil wire, or formed intoa tape about .006 inch thick and .125 inch Wide, the latter also being aform which has been employed for continuously loading signalingconductors.

The variations of permeability exhibited by two magnetic alloys,heat-treated according to one of the ordinary methods referred to above,are illustrated by the curves of Fig. 1 in which the ordinates representpermeability and the abscissae represent magnetizing forces. These'samples were in the form of 40 mil wire. The curve A desi nates an alloycontaining approximate y 7813% nickel and 211/% iron, and the curve B analloy containing'approximately 7 5% nickel and 25% iron. annealed in avacuum furnace at about 1200 C. for twenty hours, and were then allowedto cool slowly to room temperature with the furnace. The curves showthat prolonged heating of the alloys at high temperatures, followed byvery slo`w cooling, gives the alloys .high permeability, and hence suchtreatment is advantageous where this ychar` acteristic is desired. Itwill be noted, however, that with this heat treatment the permeabilityrises when the alloys are subjected to increasing Iield strengthsranging from zero to .l-gauss. The rate of change in permeability withincreasing field strengths varies-for the different compositions, butthe slope is fairly great in each case.y Slopes of this magnitude arecharacteristic of ordinary heat .treatments heretofore employed.

At a given magnetic induction and for the same permeability thehysteresis losses in alternating `magnetic fields of the magnitude here.considered are substantially proportional to the rate of variation ofthe permeability with increasing field strength.

' Reduction of the slope of curves of the sort shown in Fig. 1 isItherefore an important advantage, provided that permeability is not toomuch depressed in attaining the result. As an example of an artificewhich causes non-advantageous reduction of slope may be mentioned theintroduction of airgaps into the vmagnetic circuit. Airgaps reducepermeability to an extent which leaves 'hysteresis losses at the sameinduction substantially unaffected by the simultaneous reduction inslope of the permea- 'bility vs. ield strength curve.

The curves of Fig. 2 illustrate the greater constancy of permeabilityobtainable by means of the present invention. The curve A represents analloy containing approximately 781/% nickel and 211% iron, the curve Ban alloy containing. approximatel 75% nickel and 25% iron, and the curvean alloy containing approximately 65% nickel and 35% iron. These alloys,like those vdescribed in connection with Fig. 1, were in the form of 40mil wire. The samples were annealed in a vacuum furnace at a temperatureof about 700 C., for

, twenty-two hours, and were then allowed to cool slowly to roomtemperature with the furnace, the rate of cooling being slightly Thesealloys were less than 10 C.. per minute. The essential differencebetween Vthis heat treatment and yplotting these curves thatat low fieldv alues the hysteresis losses in the material with y 781/2%v nickel areless for the state shown in Fig. 2 byjabout 25 per cent and that thehysteresis losses in the material with nickel are less by aboutImprovements of this order are typical. l

- The samples of magnetic alloys represented by the curves of Fig. 3 arethe same in composition as those represented by the corresponding curvesof Fig. 2, but are in the formof tape .006 i-nch thick and .125 inchwide. The heat treatment was also identical except that thetemperatureem loyed was-600 C., instead of 700 C., as in t e previous case. Thecurves show that while the permeability of each of these alloys is lowerthan that obtained by employing higher temperatures, the permeability isremarkably constant` over the range of magnetizing forces up to .1gauss.

An alloy containing approximately 75% nickel and 25% iron maintained at900 C. for about twenty-one hours and cooled slow- 1y with hhafumacealso exhibited a high constant permeability as shown by the curve ofFig. 4. This vsample was in the'formof 40 mil wire. The best resultshave been obtained, according to the invention, with temperaturesbetween about 600 C., and 900 C.

While the time of heating the alloys herein describedwas in theneighborhood of twenty hours, excellent results may be obtained withtreatmentsas short as two hours the optimum temperature and duration ofthe heating being determined by trial in each case. Heating the magneticalloys at comparatively low temperatures for only a very short time,however, does not give them great constancy o f permeability. Fig. 5il-v lustrates the sloping characteristic obtained.

ioo i while in a vacuum, for two minutes. lWhen the. heating current wascut olf the tape cooled rapidly to room temperature? This heat treatmentgives the alloy a high iniin permeability with increasing fieldstrengths.

It may-be dicult in some cases to heat lthe magnetic materials forseveral hours after they are in their inal form. It has been found,however, that the valuable properties conferred by long-continuedheating at a relatively low temperature can be 4restored by a veryshort' supplementary treatment when these properties are lost byover-strain in placing the heat-treated material where it is finally tobe used. This is illustrated by the curves of Fig. 6, repre-- senting analloy which is the same in form and composition as that represented bythe curve of Fig. 5. The curve E represents an alloy containingapproximatelyv 78%76 nickel and QPL/2% iron, maintained at a temperatureof 800 C., for twenty hours and allowed to cool to room temperaturewith'the furnace at a rate slightly less than 10 C. per minute. Thiscurve shows the high and substantially constant charactcristic obtainedwith this heat treatment which is within the scope of this invention.Following this treatment the tape was overstretched about 4%. The resultis indicated by the curve F, showing that the permeability was -greatlyreduced. The overstretched tape was'then reheated at a temperature of800 C. by passing an electric current through it, while in a vacuum, fortwo minutes, this treatment restoring the material to its previous-condition as illustrated by the curve E. When another-sammon practicein nickel-iron metallurgy.

ple of this same material not previously heat-treated in accordance withthis invention was heated to 800 C., for two minutes, this being theheat-treatment according to a method referred to above, thepermeabilitywas higher but not nearly so constant, as illustrated bythecurve G. Curve G is generally` similar to the curve of Fig. 5.

Various changes'in the method and compositions described above may bemade without departing from the scope and spirit of the invention.For'example, small percentages of chromium or'other materials may beadded to the alloys to increase their resistivity, or for otherpurposes, as is com- What is claimed is: 1. The method of treating amagnetic material containing from 60% to 80% nickel and the balancechiefly iron to give it substantially constant permeability for allfield strengths up to at least .1 gauss which comprises heating it to amaximum temperature between about 600 C and about 900 C. for at leasttwo hours and cooling to 'approach room temperature at an average ratelof about 10 C. per minute. 4

.2. The method of treating a magnetic material containing from 60% to80% nickel and the balance chiefly iron to give it substantiallyconstant permeability for all field strengths up to at least .1 gausswhich comprises heating it to a maximum temperature between 600 C. and800 C. for at least two hours and coolingI to approach room temperatureat an average rate of about 10 C. per minute.

3. The method of treating a magnetic` material containing from 60% to80% nickel and the balance chiefly iron to give it substantiallyconstant permeability for all lield l strengths up to at least .1 gausswhich com- 4having substantially constant permeability for all magneticield strengths up to at least .1 gauss.

5. A.magnetic material havin substantially constant permeability for dierent magnetic field strengths up to at least .1 gauss, comprising from60% to 80% nickel and the remainderchieliy iron and a small amount ofnon-magnetic material for increasing the resistivity, said amount beinggreater than that of an impurity.

i 6. A magnetic material ,comprising approXimately 7 5% nickel and 25%iron, having substantially constant permeability for different magneticfield strengths up to at least .1 gauss. 7. A magnetic materialcomprising approximately 781/2% nickel and 211/2% iron,

having substantially constant permeability for a comparatively shorttime and cooling it to about room temperature at an average rate ofabout 10 C. er minute.

In witness whereo I hereunto subscribe my name this 29th day of OctoberA. D.,

LOUIS W. MGKEEHAN.

